STSC Excavation and Trenching Safety 5 — Questions and Answers
Question 1: What is the primary purpose of a trench shield (trench box)?
- To keep water out of the trench
- To protect workers from cave-ins by containing soil movement (Correct answer)
- To support underground utilities during excavation
- To mark the excavation perimeter for traffic control
Correct answer: To protect workers from cave-ins by containing soil movement
A trench box (shield) is designed to protect workers inside it from cave-ins, though it does not prevent the surrounding soil from collapsing.
Question 2: Sloping as a cave-in protection method for Type C soil requires a maximum slope angle of:
- 53° from horizontal (3/4:1)
- 45° from horizontal (1:1)
- 34° from horizontal (1½:1) (Correct answer)
- 26° from horizontal (2:1)
Correct answer: 34° from horizontal (1½:1)
Type C soil requires the most gradual slope of 1½H:1V (approximately 34°) because it is the least stable classification.
Question 3: When using a hydraulic shoring system, workers should:
- Install shores from inside the unprotected trench working upward
- Install shores from outside or from the top before worker entry (Correct answer)
- Install the bottom shore first while standing in the trench
- Only install shores after the full trench length is excavated
Correct answer: Install shores from outside or from the top before worker entry
Hydraulic shores must be installed from the top down so workers are never exposed to an unprotected trench face during installation.
Question 4: Which hazard is specifically associated with excavations near roadways or heavy equipment operation?
- Oxygen deficiency from soil gases
- Surcharge loading from vibration causing soil instability (Correct answer)
- Electrocution from overhead power lines
- Heat stress from confined space conditions
Correct answer: Surcharge loading from vibration causing soil instability
Vibration from traffic or equipment creates dynamic surcharge loads that can destabilize trench walls, increasing cave-in risk.
Question 5: An excavation is considered a permit-required confined space when it:
- Is deeper than 4 feet and requires a ladder
- Has limited means of entry or exit AND contains or has the potential to contain a serious hazard (Correct answer)
- Is located within 50 feet of a roadway
- Requires a trench box for protection
Correct answer: Has limited means of entry or exit AND contains or has the potential to contain a serious hazard
An excavation becomes a permit-required confined space when it is large enough to enter AND contains or may contain a hazardous atmosphere, engulfment hazard, or other serious safety risk.
Question 6: What action should a supervisor take if workers must enter an excavation where atmospheric testing shows oxygen content of 18%?
- Allow entry with standard PPE only
- Allow entry if workers wear dust masks
- Prohibit entry until ventilation raises oxygen to at least 19.5% (Correct answer)
- Allow entry for no more than 15 minutes at a time
Correct answer: Prohibit entry until ventilation raises oxygen to at least 19.5%
OSHA defines an oxygen-deficient atmosphere as below 19.5%; workers must not enter until ventilation or other means restore safe oxygen levels.
Question 7: Which document must be completed before excavation work begins in areas with suspected hazardous contamination?
- A soil boring log from a geotechnical engineer
- A site-specific health and safety plan (HASP) (Correct answer)
- An OSHA 300 log entry for anticipated hazards
- A utility strike report filed with the state
Correct answer: A site-specific health and safety plan (HASP)
OSHA 29 CFR 1910.120 requires a written Health and Safety Plan (HASP) before work at hazardous waste sites or areas with known contamination.
STSC Excavation and Trenching Safety 5
What is the primary purpose of a trench shield (trench box)?